U.S. Pat. No. 4,184,657 discloses a spring-operated cam device that operates by converting a traction force moving out of the crack into a force moving in against the wall faces of the crack through the use of cams. The friction created between the cams and the crack walls counteracts the force pulling on the device.
In general, this device consist of four cams mounted on a single shaft about which the cams can rotate, the shaft being connected to a support bar or a rod that is used to pull on the cams. The four cams are exactly the same but the two cams located at the ends of the shaft point out to one side of the shaft whereas the other two point out to the other side.
The edge of the cams that make contact with the wall forms a log spiral, the axis of which lies in the shaft. The radius of the log spiral increases gradually between two sizes, in accordance with the constant angle of the spiral. The device can thus be used in cracks sized between a minimum and maximum size limit (being this range of sizes usually referred to as “expansion range”). The edge of the cam must form a log spiral so that the limit of the coefficient of friction that enables the device to secure itself is the same in cracks of different widths into which it is inserted. The expansion range of the device can be increased with a greater constant angle of the spiral but this also increases the limit of the coefficient of friction by which the device operates, thus reducing the device's ability to secure itself.
Springs are used to extend the cams fully. The device is inserted into a crack by pulling on a trigger or an operating rod attached to cables that gather together the cams, with the end of the support bar acting as a pressing point for the user's palm of the hand when pulling the trigger and as the support point for the climbing rope. The device is then inserted into the crack and the operating rod is released. All the cams are then urged outwardly by the springs and secure themselves in position against the sides of the crack.
Patent AT-B-398.167B describes a device in which the outwardly directed load applied on the device is applied by cables. This load is exerted not on the shaft but on the cams on the side of the shaft opposite the edge that makes contact with the wall. As a result, if the cable is pulled, the cams tend to rotate towards a position in which they are extended further. Each cable is connected to each cam by a articulated link, where the articulated link is located at an eccentric position relative to the common pivot axis of the cams.
The momentum of the shaft exerted on the cam when force is applied at a distance from the shaft (i.e., at an eccentric position relative to the common pivot axis) and not directly on it can increase the range (or diameter) of the spiral or, if the range remains the same, can increase the device's ability to secure itself. However, in the different rotational positions of the cams, the distance from the shaft (the pivot axis of the cam) to the side of the crack (the point of contact between the cam and the wall of the crack) and to the vector (moment) of the force applied by the cable do not remain proportional to each other. This means that the device's ability to secure itself varies according to the different rotational positions of the cams. As for the distribution of the loads on the cams, although the loads can be distributed evenly on the cams pointing to one side and the cams pointing to the other, it is not possible to distribute the loads evenly on the two cams on one side when these are in different rotational positions. Furthermore, as the body of the device is formed by the cables that pull on the cams, these cables must be sufficiently rigid in order to take the load off the device.
It is an object of this invention to provide a cam device especially for use in climbing and which functions equally well in all the possible rotational positions of the cams.
Another object of this invention is to create a cam device wherein the force exerted on the device is distributed evenly between the cams, even when they are all in different rotational positions.